The present invention generally relates to an electrical component and more particularly, to a disc type capacitance-resistance or CR composite part for use in electrical or electronic equipment, for example, a television receiving set of a type having no transformer.
In FIG. 1 showing one example of the construction of a television receiving set of the transformerless type employing a disc type CR composite part. The television receiving set G generally includes a housing or cabinet H, a rear side cover Ha, a cathode ray tube CRT mounted on the upper front portion of the housing H for viewing, an inner chassis Ch disposed at the bottom of the housing H and serving for the grounding of electrical circuits, a tuner T accommodated in the front portion of the inner chassis Ch and having a knob K extending outwardly towards the front side of the housing H, an unbalanced type coaxial cable U drawn out from the input side of the tuner T, and an antenna connecting connector J mounted on the rear side cover Ha together with a shield chassis S having a disc type CR composite part D attached to the lower part thereof, with the end of the coaxial cable U being passed through a through-opening Do formed in the CR composite part D to be fixed thereat for maintaining the inner chassis Ch and connector J in an insulated state from each other in terms of low frequency, and in a coupled state to each other in terms of high frequency.
Conventionally, the disc type CR composite part D employed in the television receiving set as mentioned above has the construction as described hereinbelow with reference to FIGS. 2(A) to 2(C), and includes a disc-like base plate or substrate Db of ceramic dielectric material having a through-opening or through-hole Do formed at the central portion thereof, first and second electrode layers E1 and E2 respectively provided adjacent to the peripheral edge and around the through-opening Do of the substrate Db on one surface of said substrate, a pair of resistor members R provided in the diametrical direction as shown in FIG. 2(A) so as to connect for bridging the electrode layers E1 and E2, and a third electrode layer E3 provided on the other surface (FIG. 2(B)) of the substrate Db, while the electrode E3 and the first electrode layer E1 or second electrode layer E2 are shorcircuited to each other through proper shortcircuiting means, for example, by a shortcircuiting electrode Es provided in the through-opening Do as shown in FIGS. 2(A) to 2(C) or by another shortcircuiting electrode (not shown) provided on the peripheral edge of the substrate Db, or by attaching a separate metallic piece (not shown) between said electrode layers for constituting the CR composite part having an equivalent circuit as shown in FIG. 3. For actual application, electrically insulative covering layers M made of known material such as epoxy resin and the like are further applied onto the substrate Db (at least to the one surface of the substrate Db on which the first and second electrode layers E1 and E2 and the resistor members R are provided).
The known disc type CR composite part D as described in the foregoing has certain disadvantages in that its withstand voltage or voltage resistance characteristics are not very favorable. For example, upon measurements of A.C. breakdown voltage value on sample CR composite parts of the above known type (capacity .apprxeq.1800 PF and resistance .apprxeq.2 M.OMEGA.) having the dielectric substrate diameter of 20 mm.phi., and distance of 3.5 mm between the first and second electrodes E1 and E2, and having insulating covering layers of epoxy resin on the opposite surfaces of the substrate, the resultant average measured value for twenty samples was only about 3.0 KV. This value is not considered to be sufficient for withstanding the flame tests of CSA (Canadian Standards Association) standards (3 KV A.C. voltage impression for 1 minute). When each of the sample CR composite parts was carefully examined after being subjected to an A.C. breakdown voltage, it was found that the electrical discharge in said voltage impression mainly took place at the resistor members R connected in the diametrical direction between the first and second electrodes E1 and E2. The above state is considered to be attributable to the fact that, while the degree of insulation may be improved due to a comparatively favorable close adhesion between the portion of the dielectric substrate Db located between the first and second electrodes E1 and E2 and the insulative covering layer M, the adhesion between the resistor members R and insulative covering layer M is not very good, with consequent formation of fine gaps therebetween, thus resulting in reduction of the electrical insulation. More specifically, the phenomenon as described above may be ascribed to the fact that, although the surface of the dielectric substrate Db is comparatively rough, the surfaces of the resistor members R are mostly smooth.
Moreover, in the known CR composite part D as described above, since the resistor members R connected between the first electrode layer E1 and second electrode layer E2 are disposed in the diametrical direction of the substrate Db through the shortest distance on said substrate Db, the length d (FIG. 2(A)) of each of the resistor members is consequently small, making it necessary to employ a material having a high resistance value per unit area, making it difficult to obtain CR composite parts with a stable quality.